Issues collimating my laser with 4f system

[skaiser]

TL;DR Summary

Using a 4f system to shrink my collimated beam. After passage through the 4f system my beam is not properly collimated. It becomes narrow in the horizontal direction only. Vertical dimension remains nearly constant.

I am using 2 lenses in a 4f configuration. The input is a large collimated beam (632nm). After passage through the 2 lenses the beams vertical dimension remains constant, however, the horizontal dimension get smaller, as if it is being focused only in the horizontal direction. Does anyone have any suggestions of possible alignment issues I could check that could be contributing to this issue?

 

[Hyperfine]

Given the wavelength, it sounds to me as if you are running a doubled Nd:YAG. In my experience with pulsed Nd:YAG's, the output beam quality is not always the best. That is where I would start attempting to diagnose the problem.

Edit: Obviously incorrect about it being an Nd:YAG--obviously I cannot divide by 2 any longer. I would still take a very close look at the output beam spot from the LASER.

 

[DaveE]

632nm sounds like a diode laser. Nearly all high power diode lasers have different divergence in their two axes. It was probably collimated with cylindrical or aspheric lenses. Your telescope will probably also need lenses like that too. Collimation doesn't fix the divergence issues.

 

[skaiser]

DaveE- I am using a 632nm HeNe. I will describe the entire system a little more clearly..... The HeNe is sent through a spatial filter to replicate a point source very far away. This divergent source is then recollimated by a very large off-axis parabola. This is where my 4f system takes the large collimated beam as input and outputs a smaller beam my sensor array can work with. Do you think that the laser could be causing the issues with divergence in this scenario?

 

[Hyperfine]

What is the spot size, or other properties you ultimately want? The need for manipulating the output beam as you describe is far from obvious to me.

 

[DaveE]

Sorry, I think you're above my pay grade now. HeNe's should have good beam quality without asymmetrical axes. Same (probably) with your spatial filter. OTOH an off axis parabolic mirror does sound asymmetric and the result you're reporting is clearly asymmetric. Maybe @Twigg can help?

 

[Twigg]

Something sounds very wrong, and I doubt it's related to the off-axis parabolic mirror or the spatial filter. If your beam was that asymmetric before the 4f collimator, you'd be able to tell just by looking at the beam profile on a piece of paper (since it's probably huge after the spatial filter).

Is it possible that your beam is clipping somewhere in the 4f collimator? Based on what you describe, I would suggest looking for a vertical tilt or vertical offset (relative to the optical axis of your 4f collimator) in the incident beam, or a tilt in the 4f system. For large visible beams, try observing the shadow of a pointy movable object with a fixed height. I used to use a vernier height gauge for this purpose and I would observe where the shadow of the tip would be in relation to the center of the beam on a piece of paper. An alley key fastened to an optical post works just as well. I hope that helps!

As always, remember to take precautions for stray reflections when sticking things (even paper) in the beam path.

 

[skaiser]

@Twigg , thank you for the suggestions, I will give these options a try and report back!

Regards

 

[Claude Bile]

The beam was probably astigmatic prior to shrinking using the 4f telescope - it would have been less noticeable with the larger beam. I'd check the upstream optics.

 

[Redbelly98]

Sorry, late to the party here.

OP, what happens if you rotate the telescope? Do the narrow vs. unaltered directions rotate with the telescope or stay in the same orientation? That would at least tell you if the issue is with the beam or the telescope.